The long term objective of the proposed research is to characterize primary structural damage produced by ionizing radiation in RNA and DNA. The approach to this complex problem involves three steps: first, to determine what damage occurs in irradiated nucleic acid constituents, second, to ascertain how combining the constituents effects this damage, and third, to predict and identify alterations in native RNA and DNA polymers. It is the achievement of the first two steps, for a specific set of chemical systems, that constitutes the overall objective of this project. The chemical systems have been selected with the viewpoint of identifying molecular damage in the sugar-phosphate, purine and pyrimidine residues and ascertaining how the damage is modified in combinations of these components. ESR, coupled with ENDOR and isotopic labeling, will be used to study crystalline samples of: 3'cytidylic acid, 5'-deoxycytidylic acid, 5-bromodeoxyuridine, 5-chlordeoxyuridine, cytidine, 5-hydroxyuridine, uracilarabinofuranoside, inosine-5'-monophosphate, 1-methylcytosine, 1-methyluracil, 5-fluorouracil, 9-ethylhypoxanthine, and uridylyl-3', 5'-adenosine.